FIGS. 1 and 2 illustrate a traditional microwave treatment facility for treatment by producing a plasma, in particular known from document WO 01/20710 A1, which comprises:
a reactor 99 having a treatment chamber 90 (or plasma chamber) in the volume of which the plasma is produced;
several elementary plasma sources 91 each comprising an application device 92 inside the treatment chamber 90 for applying an electromagnetic wave in the microwave range; and
an electromagnetic wave generator 93 in the microwave range, connected to the application devices 92 by guide means 94 for guiding the electromagnetic wave.
During operation, the generator 93, traditionally of the magnetron type, produces an electromagnetic wave at a fixed frequency in the microwave range. For example, a magnetron 93 makes it possible to provide a variable microwave power from 0 to 2 kW at a fixed frequency of 2.45 GHz.
The electromagnetic wave delivered by the magnetron 93 is sent to a power divider 95 designed to divide the microwave power by the number k of application devices 92, generally by 2, 4, 8, 10, 12, etc. In the example of FIG. 2, the number k of application devices 92 is equal to 12.
The power divider 95 is generally made up of a rectangular waveguide in which k antennas are installed each debiting 1/k of the total power delivered by the magnetron 93. In this architecture of the power divider 95, the antennas are positioned in the guide, in which stationary waves are established, in the antinodes of the electromagnetic field. This technology is effective inasmuch as each elementary plasma source 91 behaves like a matched impedance, in other words the power reflected on each application device 92 is substantially zero such that each elementary source 91 sends, without loss, all of the debited power through the corresponding antenna.
The power debited by each antenna is then transmitted by independent guide means 94, typically of the coaxial cable type, to one of the application devices 92 through a circulator 96 equipped with a suitable water load placed at the output of the power divider 95. This circulator 96 allows the power debited by each antenna to go from the power divider toward the application devices 92, but it prevents the reflected power from going from the application device 92 to the antenna by redirecting the reflected power onto a load, in this case the water load.
The coaxial cables 94 transmit the power to the application devices 92, traditionally called applicators, through an impedance matching device 97, or tuner, place just before the corresponding applicator 92. The impedance adjustment between the plasma confined in the treatment chamber 90 and each elementary plasma source 91 is done by manually manipulating the impedance matching device 97 of each concerned line, so as to make it possible to minimize the power reflected on each applicator 92.
FIG. 3 illustrates a first example of a reactor 99a for a plasma production facility, using application devices 92 of the coaxial applicator type with an impedance matching device 97 for each coaxial applicator 92. The coaxial applicators 92 emerge in the treatment chamber 90 on the cylindrical wall of the reactor 99a. This first reactor 99a is a deposition/low-pressure etching reactor where each elementary plasma source 91 further comprises a magnetic structure 98 designed to create a magnetic field which, coupled to an electromagnetic wave with a given frequency, makes it possible to produce an electron cyclotron resonance (ECR) plasma.
In that case, the elementary plasma sources 91 are said to be elementary sources with ECR coupling or dipolar sources. The magnetic structures are traditionally made in the form of permanent magnets 98, for example formed by cylindrical magnets (magnetic dipole), positioned on the ends of the coaxial applicators 92.
This type of reactor 99a, implementing a technique for exciting the electron cyclotron resonance plasma, commonly called ECR, is particularly well suited for applications in physical vapor deposition (PVD) or plasma etching, by using a substrate holder PS that can be polarized and a target holder PC that can be polarized, which are positioned in the treatment chamber 90 on two other opposite and parallel walls of the reactor 99a. This type of reactor 99a is also suitable for plasma-assisted chemical vapor deposition (PACVD), hybrid methods combining PVD and PACVD, and reactive spraying. This type of reactor 99a typically operates at pressures below a pascal (Pa), but may go up to several thousand pascals depending on the application.
FIG. 4 illustrates a second example of a reactor 99b for a plasma production facility, using application devices 92 of the coaxial applicator type with an impedance matching device 97 for each coaxial applicator 92. The coaxial applicators 92 emerge in the treatment chamber 90 on a same wall of the reactor 99b. In this second reactor 99b, the elementary plasma sources 91 do not comprise any magnetic structure.
This type of reactor 99b is particularly well suited for applications in deposition/medium-pressure etching, for example for PACVD (plasma-assisted chemical vapor deposition) or plasma etching, by using a substrate holder PS that can be polarized positioned in the treatment chamber 90 on a wall of the reactor 99b situated across from the coaxial applicators 92. With this type of reactor 99b, the chemical deposition methods work very well in an intermediate pressure range, in the vicinity of 100 pascals (Pa), thereby making it possible to obtain high deposition speeds, but can operate more accurately from several pascals to several tens of thousands of pascals depending on the applications.
These traditional facilities for producing a microwave-excited plasma nevertheless have many drawbacks, which are also encountered in the microwave treatment facilities applied to chemistry in treatment chambers of the reactor type, agri-food in treatment chambers of the heating cavity type, etc.
A first drawback pertains to the limitations inherent to the impedance matching devices 97 to match the independence on each application device, such impedance matching devices furthermore also being used in microwave treatment facilities applied to chemistry, medicine (e.g., treatment of a part of the body, such as a tumor, by microwave radiation), or agri-food (e.g., heating or sterilization of food by microwave radiation.
In a known manner, impedance matching is a technique making it possible to optimize the transfer of power or electromagnetic energy, in the case at hand a microwave power or energy, between a transmitter, in this case the electromagnetic wave generator, and an electrical receiver called a load, i.e., the plasma confined in the treatment chamber.
Thus, as described above, in a microwave treatment facility, it is traditional to use one or more impedance matching devices situated between the application device and the electromagnetic wave generator, in order to optimize the performance. The impedance matching is said to be optimal when the power reflected by the plasma is zero, or at least as low as possible.
However, any load, such as a plasma, a chemical or gaseous reactive mixture, a solid product, etc., has an impedance that varies over time as a function of the operating conditions implemented, for example the pressure in the treatment chamber, the temperature in the treatment chamber, the nature of the gas(es) introduced into the treatment chamber to create the plasma, the proportions of those gases, the power transmitted to the load, the nature of the electromagnetic energy transmitted to the load, etc., as well as depending on the characteristics of the treatment chamber, for example the material used for its walls, its dimensions, its geometry, the surface state of its walls, etc.
Thus, the more application devices the installation has, the more complicated and restrictive the impedance matching is, in particular if each application device has its own manual impedance matching device and matching must be done for each application device and optionally for each operating condition. An impedance matching device may assume many forms and can also be integrated into the applicator.
In reference to FIGS. 2 to 4, a coaxial impedance matching device 97 with dielectric generally comprises two rings 970 concentric to the coaxial core, said concentric rings 970 being movable along the axis of the coaxial cable 94 to vary the impedance at the input of the impedance matching device 97. The concentric rings 970 constitute dielectric discontinuities which, when they are moved, make it possible to adjust the reflection coefficient. Thus, by moving the concentric rings 970, at the input of the matching device, a reflected wave is created in phase opposition with the wave reflected by the associated applicator but with the same amplitude, thus the resultant of the reflected powers is zero and the system is matched.
During a microwave treatment method, the operating conditions often change during the method, and users generally perform an average adjustment and set impedance matching devices. In this way, the impedance matching will be acceptable for the various operating conditions used during the method, but will not be optimized for each operating condition, unless the user manually redoes the matching each time these operating conditions change.
It is also known to use automatic impedance matching devices in the field of plasma production, which integrate electronic control devices controlling the movement of the mechanical elements. However, these automatic impedance matching devices are particularly complex and costly due to the electronic control devices, and not very reactive, since they require controlling mechanical elements between several positions.
A second drawback relates to the difficulty of controlling or regulating the power transmitted over each application device, or even distributing the power transmitted between the different application devices equitably; a good distribution for example favoring homogenous heating of an agri-food product or a chemical composition or mixture, in particular to favor volume targeted reactions inside a chemical reactor, or the production of a uniform plasma, in terms of volume or surface, in the treatment chamber and at a given distance from the or its walls.
In fact, this difficulty arises, inter alia, from the power dividers, which are not fully satisfactory. In the case of a magnetron generator, the greatest problem lies in the power division. In fact, the power dividers are designed to divide the microwave power at 2.45 GHz equitably between several antennas. However, the frequency of the wave emitted by the magnetron generator varies with the power, and therefore the division will only be equitable for a limited power range, which, additionally, will be different from one generator to the next.
This difficulty also comes from the application devices, which may have variable reflected powers from one application device to another. By using several application devices powered by a same generator, in some cases, it is possible to see and influence of the impedance of one application device on the other application devices, in the absence of a sufficient separation between the power supply lines for the different application devices. Imbalances are thus observed between the powers transmitted by the application devices, which harm the uniformity of the heating or the plasma in the treatment chamber.
It should be noted that inequitable distribution of the power transmitted to the load by the different application devices favors the production of homogenous heating or a uniform plasma in the treatment chamber, at least up to a certain distance from the application devices, but does not alone guarantee that such uniformity will be obtained, since that uniformity depends on the diffusion of the heating or the plasma in the treatment chamber, which in turn primarily depends, directly or indirectly, on the operating conditions (pressure, transmitted power, characteristics of the load, product or mixture to be treated, etc.) and the dimensions and shape of the treatment chamber.
In a first example facility, illustrated in FIGS. 1 and 4, the application devices are distributed in a same plane, called source plane, based on a given mesh, for example square or hexagonal. By distributing the power transmitted between these application devices equitably, a localized plasma is obtained at the end of each applicator and, by diffusion, a substantially uniform plasma is obtained in terms of density of the plasma at a certain distance from the source plane. However, moving further away from the source plane, it is possible to observe a density variation of the plasma. In this scenario, we refer to surface uniformity of the plasma, as this corresponds to a uniformity of the plasma in planes parallel to the source plane.
In the second example facility, illustrated in FIG. 3, the application devices are distributed in crowns on the cylindrical wall of the cylindrical reactor. By distributing the power transmitted between these application devices equitably, a substantially uniform plasma is obtained in terms of density of the plasma at a certain distance from the cylindrical wall. It is thus possible to obtain a uniformity over a large volume of the treatment chamber, and we then speak of volume uniformity of the plasma, since this type of facility generally operates at very low pressures; a low pressure favoring the diffusion of the species.
A third drawback relates to the difficulty of monitoring the resonance surface in the specific case of facilities using the electron cyclotron resonance plasma excitation technique.
In the presence of a uniform magnetic field B, the trajectories of the electrons are helixes wound around field lines. The electrons have an angular speed ω satisfying the following equation:ω=2π·f=e·B/m, 
where m and e respectively correspond to the mass and the charge of the electron.
When an alternating pulsed uniform electrical field ωP is superimposed on the magnetic field B, the electrons, in addition to their helical movements, undergo forces at the frequency fP=ωP/2π.
With the ECR technique, the resonance is obtained when the gyration frequency of an electron in a static or quasi-static magnetic field is equal to the frequency of the applied accelerator electrical field. In other words, for ω=ωP, the electron cyclotron resonance condition is obtained, the component of the speed of the electrons perpendicular to the magnetic field B increases, giving the electrons a helical spiral trajectory (the trajectory perpendicular to the field lines B is a spiral). Thus, a significant amount of energy is transmitted to the electrons, making it possible to ionize the neutral species of the gas easily during collisions. This type of plasma operates in a pressure range of approximately 10−3 mbar (0.1 Pa), which corresponds to a pressure low enough to allow the electrons to acquire enough energy between two collisions, but not too low for there to be enough ionizing collisions to maintain the plasma.
Thus, the creation area of the excited species depends on the magnetic field B and the frequency f of the emitted wave. However, currently, it is difficult to monitor the location of the creation zone, in other words, to monitor the resonance surface, knowing that such monitoring may have many advantages to modify the density of the plasma and therefore to optimize the performance of the installation.
The state of the art may also be illustrated by the teaching of patent application EP 1,643,641 A2, which discloses a microwave treatment facility by producing a plasma that uses a solid-state generator to excite the plasma, using an amplifier on the transmission line of the wave. This patent application describes the possibility of automatically performing impedance matching by controlling only a traditional “matching unit” matching device, with all of the aforementioned drawbacks.